Add the Packages page for nixpkgs upgrades

A page under System for router-upgrade: the channel to follow, Check,
Preview (the package versions that change, the download size, what would be
built here, a reboot note for a new kernel), Download now and Upgrade. The
steps run in router-upgrade.service, which the page asks through a request
file; the last run's output is kept in upgrade/last.log because a finished
one-shot unit has no journal to look up.

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01AeFqV9dZrK6nipHkPfyM4F

pkgs/router-webui/templates/layout.html

@@ -29,6 +29,7 @@
     </div>
     <div class="group"><span>System</span>
       <a href="/update"{{if eq .Title "Update"}} aria-current="page"{{end}}>Update</a>
+      <a href="/upgrade"{{if eq .Title "Packages"}} aria-current="page"{{end}}>Packages</a>
       <a href="/tokens"{{if eq .Title "API access"}} aria-current="page"{{end}}>API access</a>
       <a href="/advanced"{{if eq .Title "Advanced"}} aria-current="page"{{end}}>Advanced</a>
     </div>